Date of Award
Ecosystem metabolism is the coupling of carbon and oxygen through photosynthesis and respiration. Gross primary production (GPP) is the carbon fixation by photosynthesis, ecosystem respiration (R) is carbon remineralization by bacterial and plankton respiration, and net ecosystem production (NEP) is the balance. Metabolism estimates determine if ecosystem is a sink or source of carbon to the atmosphere. When a lake has a positive NEP, or the GPP:R ratio is greater than 1, it is considered autotrophic and less carbon is being lost to the atmosphere than taken in, whereas if NEP is negative (GPP:R<1) it is considered heterotrophic and loses more carbon to the atmosphere. The two main objectives of my study were to: 1) estimate metabolism using 7 years of high frequency Muskegon Lake Observatory (MLO) data and a lower frequency biological oxygen demand (BOD) light-dark bottles data and 2) to determine if there was spatial heterogeneity in metabolism across Muskegon Lake using 4 buoy sites in 2016 and 2017. The first objective showed MLO 7-year average (±SD) of GPP, R, and NEP were 0.516 ± 0.466, - 0.364 ± 0.341, and 0.028 ± 0.210 mg C L-1 d-1, respectively and the BOD 7-year average (±SD) of GPP, R, and NEP was 0.332 ± 0.226, -0.117 ± 0.069, and 0.214 ± 0.177 mg C L-1 d-1, respectively. The BUOY method consistently yielded higher rates for GPP and R and much lower rates of NEP compared to the BOD method. For the second objective, the spatial component of the study, GPP and R were significantly different across sites, but NEP was not significantly different. Our results suggest Muskegon Lake is annually a net sink of carbon. NEP may not vary much across the lake, but GPP and R and vary widely at each location. Our high frequency time-series data from multiple buoys demonstrates that freshwater lakes may display significant differences in metabolism across the ecosystem along with seasonally unequal rates of metabolism. Muskegon Lake NEP rates were comparable to NEP rates at upwelling zones in the ocean indicating more focus should be placed on inland waters when researching global carbon cycles.
Knapp, Katie Lynn, "Dynamic Carbon Cycling in Muskegon Lake – a Great Lakes Estuary" (2019). Masters Theses. 947.